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Explaining What The Heat Index Means

If you’ve been listening to our forecasts recently, chances are you’ve heard us talk about the heat index. One way we might have explained it as is “the temperature outside is 93° but it feels like 101°.”

This brings up an important question – if it feels like 101°, why not just say it is 101° instead of 93°?

Before we can answer that question, we need to answer a different question. That question is “why do we sweat when we are hot?”

Our normal body temperature is 98.6°, and sweating is a way of our bodies naturally trying to cool itself down when we get above that temperature. While sometimes it can get annoying being covered in sweat, there is science behind why our bodies do this.

When we sweat, water is released from our skin which then evaporates into the atmosphere. It takes energy to change water from its liquid phase to its gas phase, and this energy comes from our bodies. When our bodies lose energy to evaporate sweat, it also lowers our body temperature.

Humidity plays a big role in how much sweat can evaporate into the atmosphere. If dew points are higher, there is more water vapor in the atmosphere and therefore it is more difficult for sweat to evaporate compared to lower dew points and humidity.

Have you ever heard of the term “dry heat?” This is what you experience in the southwestern United States during the summer. Dew points can be as low as the 20s and 30s (compared to what we’re seeing in the 60s and 70s) which means that your body is able to cool down more effectively.

The ability for our bodies to cool via sweating is dependent on how humid the atmosphere is.

This directly affects what the heat index is.

If our bodies are not able to cool down efficiently due to higher humidity, we will feel hotter than we should.

Take a look at this table of heat index values. The x-axis is temperature and the y-axis is relative humidity. You can see that for the same temperature, higher humidity levels equal higher heat index values.

For example with an air temperature of 92°, the heat index will be 101° with a 55% humidity but it climbs to 109° with 75% humidity.

(Heat index table via the National Weather Service)

So when we say it feels like 105° when it is only 92°, it’s because high humidity is preventing our bodies from cooling down properly and therefore making it feel hotter than it is.

Brian Walder

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